A US secret military mission had "recently" tried but failed "to free a number of American hostages held in Syria", the Pentagon has revealed.
It did not say whether the US troops had also tried to rescue US journalist James Foley, a video of whose beheading by Islamic State appeared on Tuesday.
However, senior US officials - speaking on condition of anonymity - said this was the case.
IS said Foley's death was revenge for US air strikes on its fighters in Iraq.
US President Barack Obama condemned the killing as "an act of violence that shocks the conscience of the entire world".
He compared IS militants, who control large parts of Syria and Iraq to a "cancer" and said the group's ideology was "bankrupt".
Foley's mother Diane said he "gave his life trying to expose the world to the suffering of the Syrian people".
The Committee to Protect Journalists said it was "extremely concerned for all journalists" still held by IS, describing Syria as "the world's most dangerous place to be a reporter".
In a statement, the Pentagon said its operation "involved air and ground components and was focused on a particular captor network within ISIL" (the former name of IS).
"Unfortunately, the mission was not successful because the hostages were not present at the targeted location."
Meanwhile, the senior Obama administration officials said that several dozen special troops had been dropped by aircraft into Syria earlier this summer to try to rescue US hostages, including Foley.
They added that the troops had been engaged in a firefight with IS militants, killing a number of them. No Americans were killed.
James Foley, 40, was seized in Syria in 2012.
He had reported extensively across the Middle East, working for US publication GlobalPost and other media outlets including French news agency AFP.
'WARNING TO OBAMA'
The killer speaks with a British accent, as the BBC's Frank Gardner reports. Some listeners may find parts of this audio disturbing.
In the IS video, titled A Message to America, a man identified as James Foley is dressed in an orange jumpsuit, kneeling in desert-like terrain beside an armed man dressed in black.
He gives a message to his family and links his imminent death to the US government's bombing campaign of IS targets in Iraq.
Clearly under duress, he says: "I call on my friends, family and loved ones to rise up against my real killers, the US government, for what will happen to me is only a result of their complacency and criminality."
Then the masked militant - who speaks with a British accent - delivers a warning to the US government: "Any attempt by you, Obama, to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the loodshed of your people."
After he speaks, the militant appears to start cutting at his captive's neck before the video fades to black. His body is then seen on the ground.
Another captive, identified as American journalist Steven Sotloff, is shown at the end, with the warning that his fate depends on President Obama's next move.
Sotloff was abducted in northern Syria a year ago.